HDU6241 17CCPC哈尔滨L

http://acm.hdu.edu.cn/showproblem.php?pid=6241

二分判断答案，可以将原题目的限制转化为子树至多的点up和至少的点down，如果up >= down则说明当前情况可行。

 

比赛的时候觉得这种做法太简单了给Pass掉了(雾),事实上这种正确性是可以证明的，只要保证对于所有的l <= r以及l1 <= x <= r，则说明x个黑点的情况可行。

#include <map>
#include <set>
#include <ctime>
#include <cmath>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <bitset>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>
using namespace std;
#define For(i, x, y) for(int i=x;i<=y;i++)
#define _For(i, x, y) for(int i=x;i>=y;i--)
#define Mem(f, x) memset(f,x,sizeof(f))
#define Sca(x) scanf("%d", &x)
#define Sca2(x,y) scanf("%d%d",&x,&y)
#define Sca3(x,y,z) scanf("%d%d%d",&x,&y,&z)
#define Scl(x) scanf("%lld",&x)
#define Pri(x) printf("%d\n", x)
#define Prl(x) printf("%lld\n",x)
#define CLR(u) for(int i=0;i<=N;i++)u[i].clear();
#define LL long long
#define ULL unsigned long long
#define mp make_pair
#define PII pair<int,int>
#define PIL pair<int,long long>
#define PLL pair<long long,long long>
#define pb push_back
#define fi first
#define se second
typedef vector<int> VI;
int read(){int x = 0,f = 1;char c = getchar();while (c<'0' || c>'9'){if (c == '-') f = -1;c = getchar();}
while (c >= '0'&&c <= '9'){x = x * 10 + c - '0';c = getchar();}return x*f;}
const double PI = acos(-1.0);
const double eps = 1e-9;
const int maxn = 2e5 + 10;
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
int N,M,K;
struct Edge{
    int to,next;
}edge[maxn * 2];
int head[maxn],tot;
void init(){
    for(int i = 0 ; i <= N; i ++) head[i] = -1;
    tot = 0;
}
void add(int u,int v){
    edge[tot].to = v;
    edge[tot].next = head[u];
    head[u] = tot++;
}
LL l[maxn],r[maxn],up[maxn],down[maxn],sz[maxn];
void dfs(int sum,int u,int la){
    LL L = 0 ,R = 0; sz[u] = 1;
    down[u] = l[u]; up[u] = sum - r[u];
    for(int i = head[u]; ~i ; i = edge[i].next){
        int v = edge[i].to;
        if(v == la) continue;
        dfs(sum,v,u);
        sz[u] += sz[v];
        L += down[v]; R += up[v];
    }
    down[u] = max(down[u],L);
    up[u] = min(up[u],sz[u]);
    up[u] = min(up[u],R + 1);
}
bool check(int x){
    dfs(x,1,-1); bool flag = true;
    if(up[1] < x || down[1] > x) return false;
    for(int i = 1; i <= N ; i ++){
        if(up[i] < down[i]) return false;
    }
    return true;
}
int solve(){
    int l = 0,r = N;
    int ans = -1;
    while(l <= r){
        int m = l + r >> 1;
        if(check(m)){
            r = m - 1;
            ans = m;
        }else{
            l = m + 1;
        }
    }
    return ans;
}
int main(){
    int T; Sca(T);
    while(T--){
        Sca(N); init();
        for(int i = 1; i <= N ; i ++) l[i] = r[i] = 0;
        for(int i = 1; i <= N - 1; i ++){
            int u,v; Sca2(u,v);
            add(u,v); add(v,u);
        }
        Sca(M);
        while(M--){
            int p = read();LL num = read();
            l[p] = max(l[p],num);
        }
        Sca(M);
        while(M--){
            int p = read();LL num = read();
            r[p] = max(r[p],num);
        }
        Pri(solve());
    }
    return 0;
}